The present invention relates to a sensor and, in particular, to a sensor for initiating the operation of a spring-type pretensioner for a vehicle seat belt.
In some seat belt systems provided in a vehicle, such as an automobile, a pretensioner is provided to pretension the belt by operating a suitable component, e.g., by rotating the belt reel of a retractor, forming a loop in the belt itself, or pulling on a buckle, in an emergency to restrain the body of an occupant tightly on the seat by eliminating slack in the belt. As the driving mechanism for creating the pretension, a piston/cylinder operated by gunpowder has been generally used in the past.
In a pretensioner using gunpowder as the driving source, an electrical trigger is needed to activate it. This involves a problem of cost. For this reason, a spring-type pretensioner has been proposed, which operates by solely mechanical means and is less expensive.
However, in the driving mechanism for operating the pretensioner, it is generally necessary to release a large amount of energy within a limited time in an emergency, such as a vehicle collision, and to rapidly and assuredly operate the pretensioner through a full stroke with a large operating force. For this reason, a powerful spring is required when a spring is used as the driving mechanism. The preload of the spring is commensurately large, and a considerable operating force is required to release that preload. Therefore, the sensor output to operate the trigger of the pretensioner must also be large.
In the past, as one of the mechanisms to operate the trigger by sensor, there has been a system, in which one end of the lever constituting the trigger directly supports an inertial body, and the preload of the spring is released during acceleration by movement of the inertial body from the supported position by an inertia force. Such a mechanism is disclosed in Japanese Provisional Utility Model Publication 33060/1983 (Japanese Utility Model Publication 35017/1988).
However, in order to obtain sufficient trigger-operating force by a conventional type sensor as described above, the inertial body used as the means for detecting acceleration must have high inertia force, and this requires a sensor of large size and high weight.